1. Field of the Invention
The present invention relates to a subsurface safety valve used for controlling fluid flow in a well conduit and, more particularly, to an equalizing subsurface safety valve.
2. Description of Related Art
Subsurface safety valves are commonly used in wells to prevent uncontrolled fluid flow through the well in the event of an emergency, such as to prevent a well blowout. Conventional safety valves use a flapper which is biased by a spring to a normally closed position, but is retained in an open position by the application of hydraulic fluid from the earth's surface. A typical subsurface safety valve is shown and described in U.S. Pat. No. 4,161,219, which is commonly assigned hereto.
When the flapper is in the closed position, well fluid pressure below the flapper acting upon relatively large surface area of flapper makes the opening of the flapper difficult. This difficulty in opening cannot be easily overcome simply by increasing the force exerted against the flapper because the relatively small cross-sectional area of the opening piston and cylinder assembly would require a fluid pressure that may burst the control line carrying the hydraulic fluid. To overcome the difficulty in opening the flapper, different forms of mechanisms have been developed to allow the pressure above and below the flapper to equalize prior to the complete opening of the flapper. These types of safety valves are generally referred to as "equalizing" safety valves.
Additionally, when the flapper is opened the initial flow of well fluid is relatively rapid which tends to etch or erode the primary sealing surface of the flapper. Any damage to this primary sealing surface is extremely critical because it is this sealing surface which must be in tact to prevent uncontrolled flow of well fluids and to prevent a possible well blow out.
U.S. Pat. No. 3,078,923, which is commonly assigned hereto, discloses a through-the-side wall equalizing valve mechanism which is opened by the downward movement of the flow tube prior to the flow tube contacting and opening the flapper. While the initial fluid flow is beneficially directed across the equalizing valve mechanism to keep the flapper's sealing surface undamaged, the equalizing valve mechanism is subject to the same relatively rapid fluid flow which will erode its valve sealing surface. Again, any damage to any sealing surface must be avoided for the safety valve to be fully functional in order to protect the well. Additionally, if the spring that holds the valve mechanism in place becomes damaged or lost, the valve mechanism may become off-center or simply fall out of its seat. Then, the safety valve would be non-functional because an uncontrolled opening would be created which could not prevent the well fluid from flowing therepast.
U.S. Pat. Nos. 4,427,071 and 4,457,376 each disclose an equalizing mechanism which consists of a flapper with an inclined upper surface. When the flow tube is extended to open the flapper, the lower edge of the flow tube contacts the inclined surface to cause the flapper to partially unseat the flapper prior to it being fully opened. While this equalizing mechanism does not have the spring retention problem mentioned above, it still has the problem of the initial fluid flow damaging the primary sealing surface of the flapper.
U.S. Pat. Nos. 4,415,036 and 4,478,286 each disclose an equaling mechanism which consists of a plug valve held by a spring across a vertical opening in the flapper itself. While the plug valve in U.S. Pat. No. '286 does not have the problem of the initial fluid flow eroding a sealing surface, both plug valves shown in these patents must be held by a spring that can be damaged or simply fall off due to the actions of corrosive fluids over a long period of time. Again, if the spring is lost, the plug valve will fall out of its opening in the flapper. Then, the safety valve would be non-functional because an uncontrolled opening would be created which could not prevent the well fluid from flowing therepast.
U.S. Pat. Nos. 4,629,002, 4,703,805, 4,722,399 and 5,058,682, all of which are commonly assigned hereto, each disclose equalizing mechanisms which do not address the problems of erosion of the primary sealing surface and of spring retention. Each of these patents disclose fluid flow diverting arrangements, generally referred to as labyrinth passages, to slow the initial fluid flow to prevent sealing surface erosion. In spite of these safety valves' performance benefits, these safety valves require complex machining operations and numerous parts which add significantly to their costs of manufacture.
There is a need for an equalizing safety valve which can be relatively inexpensively manufactured, and which does not suffer the problems of primary and equalizing sealing surface erosion and of spring retention.